(a) Field of the Invention
The present invention relates to a display device and a driving method thereof. More particularly, the present invention relates to a light emitting display device and a driving method thereof.
(b) Description of Related Art
Recently there has been a trend toward production of lightweight and thin personal computers and television sets. To support the above-mentioned trend, lightweight and thin display devices are required. Flat panel displays are lightweight and thin in comparison to conventional cathode ray tubes (CRTs) and thus are widely being substituted for CRTs.
Examples of flat panel displays include a liquid crystal display (LCD), field emission display (FED), organic light emitting display (OLED), plasma display panel (PDP), and others.
Generally, an active matrix flat panel display includes a plurality of pixels arranged in a matrix. Active matrix flat panel displays display images by controlling a luminance of the pixels based on given luminance information. An OLED is a self-emissive display device that displays images by electrically exciting light emitting organic material. The OLED consumes less power than many other flat panel displays, and has a wide viewing angle and fast response time, thereby making the OLED advantageous for displaying moving images.
A pixel of the OLED includes a light emitting element and a driving thin film transistor (TFT). The light emitting element emits light having an intensity that varies in response to a current driven by the driving TFT, which in turn varies in response to a threshold voltage of the driving TFT and a voltage between gate and source of the driving TFT.
The driving TFT includes polysilicon or amorphous silicon. Although a polysilicon TFT has several advantages, disadvantages of the polysilicon TFT include a complexity of manufacturing polysilicon, which thereby increases manufacturing costs. In addition, it is difficult to make a large OLED employing polysilicon TFTs.
On the contrary, an amorphous silicon TFT is manufactured using fewer process steps than the polysilicon TFT. Thus, making a large OLED is easier to accomplish using amorphous silicon TFTs than using polysilicon TFTs. However, an OLED employing amorphous silicon TFTs exhibits a deterioration of bias stress stability. The deterioration of bias stress stability is indicated by a reduction of an output current of the amorphous silicon TFT over time during a long-time application of high DC control voltages with high driving voltages. The deterioration of bias stress stability causes a luminance of the OLED to be varied for a given data voltage.